Inverters are used to convert the power generated by a generator, for example by a photovoltaic generator, as a DC voltage into an AC voltage power compliant with a power supply grid. In this case, the losses inside the inverter result in heating of the inverter, in particular heating of the power semiconductors as switching elements, which may be damaged as a result of excessive heating. It is therefore necessary to monitor the temperature of these switching elements and to reduce the converter power of the inverter, if necessary, if a limit value of the switch temperature is exceeded. For this purpose, the modules inside the inverter which contain the power semiconductors have monitoring elements for the temperature, for example thermodiodes or NTC (Negative Temperature Coefficient) resistors. A load limiter as part of the inverter monitors the temperature measured in this manner and initiates a reduction in the converter power if the temperature of the monitored switches is expected to be exceeded upon continuation of inverter operation with the instantaneous converter power. Accordingly, it can be ensured that the modules are protected from excessive heating, in particular from heating, that would shorten the service life or destroy switching elements.
The disadvantage of the known prior art is that switching elements with no associated monitoring elements for the temperature cannot be monitored in the above-described manner. Instead, it is necessary to use thermal modeling of the inverter to calculate the temperature of the latter indirectly via the recorded measurement variables, for example the temperature of the switches of an output bridge of the inverter, and the instantaneous converter power and to convert it into limit values requiring a reduction of the converter power. Such indirectly monitored switching elements may be, for example, part of a boost converter converting the DC voltage provided by the connected generator into a higher value of a voltage across an intermediate circuit of the inverter. This results in the situation in which the inverter is limited, that is to say the instantaneous converter power is reduced to a value equal to or below a calculated limit value even though no critical temperatures would yet be reached if operation of the inverter were continued with a converter power which has not been reduced. The cause of this is that it is not possible to reliably infer the temperature of the switching elements of the boost converter from the measurement variables of the instantaneous converter power and the bridge temperature because measurement variables that significantly influence the load on the switching elements of the boost converter are not taken into account. In this situation, often so-called worst-case scenarios are used, in which values which correspond to the most unfavorable operating state are assumed for measurement variables that have not been determined. Consequently, the converter power is therefore reduced too early in this case if the actual operating state does not correspond to this most unfavorable operating state.
This disadvantageous effect occurs, in particular, when the switches that are thermally directly monitored using temperature sensors and the indirectly monitored switches are arranged at a great distance from one another in the inverter and therefore have an unreliable temperature correlation. The safety margins for the worst-case scenarios are then particularly large and the inverter is often unnecessarily limited. This situation is pronounced in high-frequency converters, for example, since the DC isolation between the boost converter as the input stage and the output bridge as the output stage regularly also entails spatial separation. An additional outlay in terms of design likewise results when electrical signals from DC-isolated regions are needed to monitor a switch.
Even if the modules with the power semiconductors have temperature sensors, the measurement variable determined in this manner provides only an inaccurate measure of the load on the individual switch, in particular if the loads on the individual switches of a module are different, and there is dependence on a sufficiently large safety margin when determining the load threshold for limiting the inverter according to the known methods in accordance with the prior art.